Adding Usage and Titled interfaces

[kenshaw]

Adds Usaged and Titled interfaces for allowing better flexibility for
controlling how command usage is displayed, and adds accompanying
documentation in README.md.

[coveralls]

Coverage decreased (-0.8%) to 93.096% when pulling b6ed360 on kenshaw:add-usaged-and-titled-interfaces into f7c0423 on alexflint:master.

[alexflint]

Thanks for this pr @kenshaw. I'm somewhat hesitant to merge the change because I'm extremely keen to keep the API from growing, even though I understand this is mostly a transparent change. My instinct is to instead expand the Parser struct to support printing just the arguments and options, so that folks can customize the usage string more like this:

p, err := arg.NewParser(os.Args)
...
if err = p.Parse(&dest); err != nil {
  fmt.Println("Some title: some usage string")
  p.PrintPositionals()
  p.PrintArguments()
}

[kenshaw]

@alexflint OK -- thanks for the consideration. I thought you might be hesitant to add this. If there's something that can be changed in order to support this, let me know what I can do, and I'll gladly change it.